Catalysts that have been considered to be effective are those as disclosed in “JOURNAL OF CATALYSIS” (1997, vol. 165, pp. 150-161) and Japanese Patent Provisional Publications Nos. 10-272366 and 11-60514, in which the catalysts are capable of producing hydrogen and aromatic compounds such as benzene from methane in one stage.
However, the coke formation is still serious even if such catalysts are used, with which the catalytic performance declines in a short time. Additionally, there exists a problem that a methane conversion rate (or a ratio of methane used to produce aromatic compounds and hydrogen) is low.
In view of the above, some processes for solving the above problems are disclosed in Patent Documents 1 to 9 and the like, in which a lower hydrocarbon aromatization catalyst on which molybdenum or a metal component consisting of molybdenum and other is loaded produces aromatic compounds from methane. According to such processes, reaction is made with the addition of hydrogen, carbon dioxide or carbon monoxide and therefore coking (carbon formation or methane-decomposing reaction) can be suppressed so as to maintain the catalytic activity for a long period of time. Particularly in a catalyst-producing process as disclosed in Patent Document 6, a metallosilicate is subjected to a surface treatment with silica so that the catalytic activity is stabilized.
However, a further development in such catalysts is desired in order to further improve the production efficiency of aromatic compounds and hydrogen, and additionally drawbacks have been encountered in the above conventional techniques. Even with the above conventional techniques, the formation rate of aromatic hydrocarbons (such as benzene) is currently not so high as to perfectly suppress the coke formation that gradually develops with a lapse of time. More specifically, a decrease in formation of aromatic hydrocarbons particularly benzene) is grown.    Patent document 1: Japanese Patent Provisional Publication No. 2005-254122    Patent document 2: Japanese Patent Provisional Publication No. 2005-254121    Patent document 3: Japanese Patent Provisional Publication No. 2006-263682    Patent document 4: Japanese Patent Provisional Publication No. 2005-254120    Patent document 5: Japanese Patent Provisional Publication No. 2006-263683    Patent document 6: Japanese Patent Provisional Publication No. 2007-014894    Patent document 7: Japanese Patent Provisional Publication No. 2005-255605    Patent document 8: International Publication No. WO05/028105    Patent document 9: International Publication No. WO06/011568